Typically, a planetary gear train is realized by combining a plurality of planetary gear sets and friction members. It is well known that when a planetary gear train realizes a greater number of shift speeds, speed ratios of the planetary gear train can be more optimally designed, and therefore a vehicle can have economical fuel mileage and better performance. For that reason, the planetary gear train that is able to realize more shift speeds is under continuous investigation.
Though achieving the same number of speeds, the planetary gear train has a different operating mechanism according to a connection between rotation elements (i.e., sun gear, planet carrier, and ring gear). In addition, the planetary gear train has different features such durability, power delivery efficiency, and size depend on the layout thereof. Therefore, designs for a combining structure of a gear train are also under continuous investigation.
If the number of shift-speeds, however, increases, the number of components in the automatic transmission also increases. Therefore, mountability, cost, weight, and power delivery efficiency may be deteriorated.
Particularly, since the planetary gear train having a number of components is hard to be mounted in a front wheel drive vehicle, research for minimizing the number of components has been developed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.